Carausius, AE antoninianus, London.

Obv: IMP C CARAVSIVS PF AVG, radiate, draped and cuirassed bust right.
Rev: PAX AVG, Soldier, semi-naked, chlamys hanging from
his right shoulder, standing right, holding spear, point 
downwards and round shield before his chest.
S-P across fields.
Mintmark ML.

RIC V-2, London 134; Webb 158; Cohen 236.
Extremely rare.
Although RIC gives this coin only an R2 rating, there is
only one of its kind known, submitted to Num. Chronicle 
by the owner in 1852.
The coin is cited by Cohen, who is cited by Webb, who is
cited by RIC. 
Because of its rarity (neither Webb, RIC nor Cohen show
an image), I am using the image of the original cast from
NC 1852.